1. Field of the Invention
The invention relates to a steering control system.
2. Description of the Related Art
A steering system configured to assist a user's operation of a steering wheel is described in, for example, WO 2012/133590. WO 2012/133590 describes a steering system that adjusts a rotation angle of a pinion shaft of a rack-and-pinion mechanism to a pinion angle command value through feedback control (hereinafter, referred to as “rotation angle feedback control”). The rack-and-pinion mechanism mechanically couples a steering wheel of a steering mechanism of a vehicle to steered wheels. Through the rotation angle feedback control, the steering system described in WO 2012/133590 achieves favorable steering characteristics while reducing reverse input vibrations transmitted from the steered wheels, when assisting the user's operation of the steering wheel.
Further, the steering system described in WO 2012/133590 calculates a road information controlled variable as a compensation component for increasing or decreasing a controlled variable of the rotation angle feedback control, based on a vehicle speed value that is a detected value obtained by a vehicle speed sensor. Through the rotation angle feedback control, the rotation angle of the pinion shaft is controlled to be adjusted to a pinion angle command value calculated based on an ideal model for a rotary shaft that rotates in response to a load (steering torque) applied to the steering wheel, in order to reduce the reverse input vibrations transmitted from the steered wheels. Thus, for example, when the vehicle is stationary, the steering system described in WO 2012/133590 offsets the controlled variable of the rotation angle feedback control with the road information controlled variable, thereby substantially disabling the rotation angle feedback control. This allows the user to appropriately perform an operation of the steering wheel (so-called stationary steering (i.e., dry steering being the act of turning the steering wheel of a vehicle while the vehicle is stationary)).
In the steering system described in WO 2012/133590, when a vehicle speed value is abnormal, the accuracy of the compensation component for increasing or decreasing the controlled variable of the rotation angle feedback control may be reduced. For example, due to abnormality of a vehicle speed value, the vehicle may be regarded as being traveling although the vehicle is actually stationary. This prevents the rotation angle feedback control from being substantially disabled. That is, when the vehicle speed value is abnormal, there is a possibility that the user cannot appropriately perform an operation of the steering wheel.